What

Concurrency control method commonly used by database management systems to provide concurrent access to the database and in programming languages to implement transactional memory.

How

Each SQL statement sees a snapshot of data (a database version) as it was some time ago, regardless of the current state of the underlying data.

Why

This prevents statements from viewing inconsistent data produced by concurrent transactions performing updates on the same data rows, providing transaction isolation (ACID) for each database session.

The main advantage of using the MVCC model of concurrency control rather than locking is that in MVCC locks acquired for querying (reading) data do not conflict with locks acquired for writing data, and so reading never blocks writing and writing never blocks reading.